Euglena do photosynthesis using the same
basic process that plants use. They also move
around and eat, as do animals. But you are
right that they are not classified as either of
these. That is because they are
unicellular. “Uni” means one (a unicycle has
one wheel). In order to be classified as a
plant or animal, an organism has to be
multicellular, or made of more than one cell.
Since it is a unicellular organism with some
plant and animal characteristics, it is called a
protist. Plant cells have walls. There’s no
cell wall around a Euglena’s cell membrane, so
it is a protozoan. It used to belong to the
kingdom Protista. Now most scientists think we
should stop using that kingdom.

Keep in mind that humans make classifications
in order to organize the amazing complexity of
the world. The natural world does not fit
neatly into all of our classifications. We can
be pretty sure about how closely related things
are by analyzing their DNA, but there’s no magic
number that means they belong in the same
kingdom, order, or family.

If you want a real challenge, read up on
Volvox. It can do photosynthesis. It can live
singly or in a colony. It can swim around with
flagella. Is it a plant, an animal, an algae, a
protist? You make the call.

Thanks for asking,

Answer 2:

Most species of Euglena have
photosynthesizing chloroplasts within the body
of the cell, which enable them to feed by
autotrophy (making energy-containing organic
molecules from inorganic raw material through
the use of an energy source such as sunlight),
like plants. However, they can also take
nourishment heterotrophically (making use of
food that comes from other organisms in the form
of fats, carbohydrates, and proteins), like
animals. Since Euglena have features of both
animals and plants, early taxonomists, working
within the old two-kingdom system of biological
classification, found them difficult to
classify - not because they had features
different from an animal and different from a
plant, but because they had some features that
were animal-like and some features that were
plant-like. So could they be both? It was
decided that they could not, and in fact other
species that had both animal and plant-like
characteristics were discovered and so in 1866
Ernst Haeckel added a third kingdom to the
Animale and Vegetabile of the old taxonomic
system: the Kingdom Protis

Answer 3:

Plants are multicellular organisms with
cellulose cell walls. Depending on the
definition, plants may or may not need also to
enclose the developing new plant in an embryo
(if this definition holds, then green algae are
not plants; otherwise, they are).

Animals are multicellular organisms held
together by a specific protein called
collagen.

Euglena is single-celled, and the cell is
enclosed in a semi-rigid protein sheath, not a
true cell wall but not a simple cell membrane.
Euglena is entirely unicellular, has no collagen
and no cellulose, stores energy in paramylon
bodies (not starch as plants do).

Euglena is photosynthetic, but the origin of
Euglena's chloroplasts is taken *from* a green
alga, not directly from
cyanobacteria/chloroxybacteria as plants and
green algae are.

Andy Simpson

Answer 4:

From Wikipedia, Euglena is a genus
of "unicellular flagellate protists." The key to
why they're not considered plants or animals is
in the word "unicellular," which means the
entire organism consists of one cell. This is
very different from a plant or an animal, both
of which are comprised of many
different "specialized" cells that carry out
different functions!